README.md

Lesson 10 — Shared Library

What you'll learn

How to build a shared library (.so on Linux, .dylib on macOS), link against it, and handle runtime library loading.

Static vs shared — what's different?

Aspect	Static (.a)	Shared (.so/.dylib)
When code is included	At link time (copied into executable)	At runtime (loaded from disk)
Executable size	Larger (contains library code)	Smaller (just a reference)
Updating the library	Must re-link the executable	Just replace the .so/.dylib
Runtime requirement	None — self-contained	Library file must exist on disk

Key compiler flags

-fPIC — Position Independent Code

Shared libraries can be loaded at any memory address. The code must be compiled so it works regardless of where it's placed:

CXXFLAGS = -std=c++20 -Wall -Wextra -Iinclude -MMD -MP -fPIC

-shared — produce a shared library

$(LIB): $(LIB_OBJS)
	$(CXX) -shared -o $@ $^

-rpath — embed the library search path

At runtime, the OS needs to find the .so/.dylib. Without -rpath, you'd have to set LD_LIBRARY_PATH (Linux) or DYLD_LIBRARY_PATH (macOS) every time. -rpath embeds the search path in the executable:

# macOS: look next to the executable
RPATH_FLAG = -Wl,-rpath,@executable_path

# Linux: look next to the executable
RPATH_FLAG = -Wl,-rpath,'$$ORIGIN'

Platform detection

Library extensions differ by platform. This Makefile detects the OS:

UNAME := $(shell uname)
ifeq ($(UNAME),Darwin)
    LIB = libstrutil.dylib
    RPATH_FLAG = -Wl,-rpath,@executable_path
else
    LIB = libstrutil.so
    RPATH_FLAG = -Wl,-rpath,'$$ORIGIN'
endif

$(shell uname) runs a shell command and captures the output. := means "evaluate immediately" (not lazily).

Try it

cd 10_shared_library
make                        # builds libstrutil.dylib (or .so) and app
./app                       # upper, lower, reverse transformations
otool -L app 2>/dev/null || ldd app    # see runtime library dependencies
make clean

Files

File	Purpose
include/strutil.hpp	Declares to_upper(), to_lower(), reverse()
src/strutil.cpp	Implements them
src/main.cpp	Uses the library
Makefile	-fPIC, -shared, platform detection, -rpath